Particle size contributions to bulk magnetic susceptibility in Chinese loess and paleosol

The magnetic enhancement of paleosol is observed in the Chinese loess. The origin of this magnetic enhancement is still very uncertain. It is, however , a key problem in correctly understanding the paleoclimatic significance o f changes of magnetic susceptibility and in transferring the magnetic signa ls to paleoclimatic parameters. Two main models have been proposed to expla in the mechanism of magnetic enhancement in paleosols: a depositional model and a pedogenic model. Together with composition and concentration, grain size distribution of magnetic minerals also plays an important role to the magnetic enhancement of paleosols. Systematic susceptibility measurements w ere carried out on the samples of the upper part (S-0 to top of L-2) of thr ee loess sections from Jixian, Xifeng, and Xining, along an east-west trans ect in the loess plateau, China. The samples with the highest value of magn etic susceptibility in S-1 and the lowest value in L-2 of each section were selected as the representatives. These representatives were separated into different grain size fractions based on Stokes' law for coarse grains and by centrifuge for fine grains. Measurements of magnetic susceptibility and mass have been carried out on these fractions. Results show that for the lo esses magnetic susceptibility changes little in the fractions with differen t grain size and for the paleosols it increases with decreasing grain size. The magnitude of changes is bigger in the east (Jixian and Xifeng) than th at in the west (Xining). The fraction with the finest particle size in pale osols does not show very high magnetic susceptibility values. A new approac h is used to estimate the contribution of each fraction to total magnetic s usceptibility. The contribution comes mainly from coarse grains (>10 mu m) for loess samples. It reaches about 90%. The main contribution comes, howev er, from the particles with medium size (10-0.2 mu m). The very fine graine d particle(< 0.3 mu m), which is considered to be with the pedogenic origin , contributes little to the bulk magnetic susceptibility, no more than 3% b ecause of their very little amount. This approach provides a sounder basis for the study of the origin of the magnetic susceptibility enhancement in p aleosols and of the paleoclimatic significance of magnetic susceptibility o f loess and paleosols. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.